This invention relates to a system including a storage subsystem and a server subsystem, and more specifically to a snapshot management technique.
SAN-NAS integrated storage systems which high-performance SAN and easy-to-manage NAS are lately becoming popular.
SAN-NAS integrated storage systems have a snapshot function in order to protect data stored in a primary volume. Using the snapshot function, a SAN-NAS integrated storage system takes a snapshot which is a still image of a primary volume at a specific point in time. Known snapshot technologies include full-copy snapshot and differential snapshot.
U.S. Pat. No. 6,101,497 discloses a technique related to full-copy snapshot. A SAN-NAS integrated storage system creates a mirror volume synchronized with a primary volume. The SAN-NAS integrated storage system creates a full-copy snapshot by performing splitting processing in which the created mirror volume is split physically.
A drawback of full-copy snapshot is large consumption of disk resource, which means that fewer generations are provided. An advantage of full-copy snapshot is very high access performance owing to physical separation from primary volumes. Full-copy snapshot is therefore used as snapshot of which high access performance is demanded, for example, as snapshot for online backup.
US 2004/01896900 discloses a technique related to differential snapshot. A SAN-NAS integrated storage system stores only differential data which is the difference between a differential snapshot and a primary volume. The SAN-NAS integrated storage system combines the stored differential data with the primary volume, to thereby provide a virtual differential snapshot.
Created by combining differential data with a primary volume, differential snapshot does not have as high access performance as full-copy snapshot. On the other hand, differential snapshot uses a small amount of disk resource and therefore can provide more generations than full-copy snapshot. For instance, differential snapshot is employed as snapshot for regular backup of a shared file server.
The full-copy snapshot and differential snapshot functions enable SAN-NAS integrated storage systems to provide snapshots suited to access performance requirements and disk resource usage requirements.
JP 2005-267569 A discloses a storage system that utilizes a snapshot function in remote copy. This storage system copies differential data from a primary volume to a copy volume while referring to a bitmap.